1. Field of the Invention
The present invention relates generally to discriminator apparatus and more particularly, to an improved low noise phase lock discriminator circuit having particular utility for use in testing the spectral purity of rf signals.
2. Description of the Prior Art
Phase-locked circuits have long been used for various purposes in the communications industry as evidenced by the U.S. Pat. Nos. to Sui, 3,624,511; Brooks, 3,316,497; Widl, 3,287,657; Gruen, 2,838,419; Hileman, 3,363,194; Deman, 3,320,544; Perkins et al., 3,495,184; Taylor, 3,421,105; and Haggai, 3,611,168. However, the field of use of such circuits is limited to certain applications unless very sophisiticated reference sources are used to insure that the reference noise is low. For example, an ordinary phase-locked circuit is not suitable for use in making highly accurate measurements of the spectral purity of an rf signal because the sensitivity of the apparatus is limited by the inherent noise characteristics of the detector reference source.